Astronomy 101 for the Post

"All the planets in the solar system orbit the sun in a circle. Not Sedna. All the planets orbit in the same plane. Sedna's orbit is canted 12 degrees."

Editor's note:I have a few problems with this paragraph -as it appears in the Washington Post.

A precise definition of a circle is "A plane figure in which all the points are the same distance from a point called the center." All planetary orbits depart from such a definition - some more than others. Indeed Kepler discovered that orbits are indeed elliptical. How far an orbit departs from being a perfect circle is refered to as its eccentricity. Pluto's orbit is highly eccentric with its distance to the sun ranging from 2.7 billion to 4.6 billion miles. Indeed, Pluto is often closer to the Sun than the planet Neptune - as it was for 20 years between 1979 and 1999.

Pluto's orbit is also inclined by 17 degrees with respect to the plane of the solar system (ecliptic) which uses Earth's orbit as a central reference point. Pluto's orbit is more inclined from the ecliptic than Sedna's. All other planets are much, much closer to the plane of the ecliptic than Pluto - but all still deviate from it somewhat. Indeed, Mercury's orbit is inclined by 7 degrees (Sedna's is inclined by 12) and Mercury's eccentricty, at 0.21, is very close to Pluto's (0.25) whereas Sedna's is greater at 0.86.

The solar system is not as circular - nor as flat - as the author would suggest it to be.

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